Beauveria bassiana

Beauveria bassiana is a fungus which causes a disease known as the white muscadine disease in insects. Insects are attacked at larval or adult stages. When spores of this fungus come in contact with the cuticle (skin) of susceptible insects, they germinate and grow directly through the cuticle to the inner body of their host. Here the fungus proliferates throughout the insect's body, producing toxins and draining the insect of nutrients, eventually killing it. Therefore, unlike bacterial and viral pathogens of insects, Beauveria and other fungal pathogens infect the insect with contact and do not need to be consumed by their host to cause infection. Once the fungus has killed its host, it grows back out through the softer portions of the cuticle, covering the insect with a layer of white mold (hence the name white muscadine disease). This downy mold produces millions of new infective spores that are released to the environment. (CT IPM)

Current commercial formulations are limited to those produced by two companies:

All are foliar formulations of the spores of the fungus, and as the spores are microscopic, can be applied with standard spray equipment. BotaniGard®22WP can also be used as a dip for cuttings, and for soil applications - landscape and containers.

Mode of Action: B. bassiana produces spores that are resistant to environmental extremes and are the infective stage of the fungal life cycle. The spores infect directly through the outside of the insect's skin. Under favorable temperature and moisture conditions, a conidium (singular of "conidia”) adhering to the host cuticle will germinate. The fungal hypha growing from the spore secretes enzymes which attack and dissolve the cuticle, allowing it to penetrate the skin and grow into the insect body. Fungal spores are readily killed by solar radiation and infect best in cool to moderate temperatures (Goettel et al., 2000, Wraight and Ramos 2002). Once inside the insect it produces a toxin called Beauvericin that weakens the host's immune system. After the insect dies, an antibiotic (oosporein) is produced that enables the fungus to outcompete intestinal bacteria. Eventually the entire body cavity is filled with fungal mass. When conditions are favorable the fungus will grow through the softer parts of the insect's body, producing the characteristic "white bloom" appearance. Relative humidity must be 92% or more for B. bassiana to grow outside the insect. These external hyphae produce conidia that ripen and are released into the environment, completing the cycle.

Application Tips: High humidity and water amplify the activity of the conidia and the infection. Fungal spores are readily killed by solar radiation. It is best to spray the plants with the anthropoid pests in the morning or late afternoon, in cool to moderate temperatures (Goettel et al. 2000, Wraight and Ramos 2002). Apply the Beauveria bassiana liquid spray to the top as well as the undersides of the leaves or wherever the arthropod primarily occurs. Good coverage is a must. The spores have a relatively short life cycle, so it is important that the spray has sufficient opportunity to contact the insect. For insects that bore into a plant, control is difficult. For best results, applications should be made during the early growth stages of the insect before much damage has occurred. Speed of kill depends on the number of spores contacting the insect, insect age, susceptibility and environmental conditions.

Phytotoxicity has been minimal, but a small sample of targeted plants should be checked. After mixing with water, Beauveria products should be sprayed as soon as possible, as fungal spores die and material loses its viability overnight. The foliage should be sprayed until the plants are wet thoroughly, but not to run-off. Equipment that gets the material to the undersides of the leaves will result in prolonged activity, as spores are inactivated by sunlight. Evening applications may be desirable.

The frequency of applications also depends on the pest and the crop. For greenhouse pest problems, applications every 5-7 days are recommended. Again, all applications should be based on monitoring of pest populations.

The rate at which Beauveria spores kill their host is dependent on temperature. At a constant 72 oF, small potato beetle larvae are killed in 3-5 days. Under field conditions in Maine, it may take 7 to 10 days to kill larvae.

Impact on Beneficial Insects: Since this product is used to control a broad range of insect types (including beetles and ants), predators in these insect classes, such as ladybugs, could also be affected. Caution should be used when applying it when honeybees are actively foraging.